1. Field of the Invention
This invention relates to the field of conductors. More specifically, the invention comprises a structure and method for insulating high-temperature superconductor tapes that electrically insulates the conductors while mechanically decoupling them from the much-stronger encapsulant.
2. Description of the Related Art
High-temperature superconductor tapes (“HTS tapes”) are now used in many applications, including the creation of electromagnetic windings. Such tapes generally include a strong mechanical substrate (such as Hastelloy) bonded to a thin layer of superconductor material. An electrically insulating layer must be added over the assembly. An encapsulant layer must also be provided.
Many types of HTS tapes are known, with one example of a suitable superconducting material being YBa2Cu3O7-δ (“YBCO”). YBCO-based HTS tapes show great potential for the construction of magnets having a very high field strength. Materials presently used in superconducting magnets (such as Nb—Ti and Nb3Sn) cannot operate in fields exceeding about 30 Tesla. In contrast, YBCO HTS tapes retain a superconducting state in magnetic fields well above 100 Tesla.
Construction of superconducting magnets requires not just a suitable conductor, but also a suitable insulation to resist over-voltages during quenching. An encapsulant is also required. This component prevents tape movement, delamination, and resulting damage during encapsulant curing, thermal cycling, and actual operation of the magnet. The magnet is operated at low temperatures using a cryogenic coolant such as liquid nitrogen or liquid helium. The coolant maintains the temperature needed that the conductor remains in a superconducting state.
To date, conductor insulations are wrappings of various adhesive-backed tapes (such as polyimide or polytetrafluoroethylene) and coatings of various polymers (such as varnish, epoxy, and acrylates). If a conductor is created by wrapping one of the insulation materials in a helical manner, gaps are inadvertently introduced. These gaps create variations in the electric field. In addition, the thinnest wrapping materials currently available are far thicker than would be optimum. This is particular true when the wrapping is overlapped to eliminate gaps. The result is a reduction in the coil-winding current density, which increases the size and the cost of the coils.
Insulation with a thickness of about 10 μper tape side is preferred. Polymer coatings of the tape conductors tend to be non-uniform, due primarily to the rectangular cross section and high aspect ratio of the tape (around 40 to 1). The superconducting tape itself has a nominal thickness of only about 100 μm. The edge of the tape is thus quite thin, which causes problems during the application of a coating polymer. The surface tension of such a polymer when in the liquid state is in opposition to the dynamic viscosity. The coating tends to pull away from the tape edges and leave them exposed.
An encapsulant is typically added over the exterior of the HTS tape assembly. Paraffin or epoxy is commonly used. These are electrically insulating and—in principle—they should eliminate the need for a separate insulating material adjacent to the conductors. However, a prohibitively labor-intensive process would be required to electrically isolate the coil windings and layers during encapsulation. Furthermore, paraffin is temperature sensitive, has a high vapor pressure, cracks, and cryoblisters. Although epoxy is an effective encapsulant, it can cause stress-induced damage to the conductor by a strong mechanical coupling to the conductor (either by direct adhesion to the conductor or by adhesion through the various adhesive-backed tapes or polymer coatings).
Detrimental thermal and electromagnetic tensile and shear stresses are introduced at the boundary between the conductors and the encapsulant during both cooling and energization of the magnet. The thermal stresses in a magnet are produced by differential thermal contraction during cooling. The electromagnetic stresses are those produced by the interaction of the self-field of the magnet and its energizing current (the so-called Lorentz stresses).
In a solenoidal coil, a hoop stress develops parallel to the conductor axis in the winding. Additionally, axial compressive stresses develop that tend to expand the coil outward. The existing coated conductor designs are strong in tension along the tape axis. However, they are much weaker when stress is applied in a direction that is perpendicular to the tape axis. It has been observed that the ratio of axial to perpendicular strength in HTS tape may be as high as 100 to 1. This characteristic allows delamination of the superconducting YBCO thin film from its Hastelloy substrate at stresses as low as about 5 MPa. The present invention seeks to mitigate this damaging phenomenon by using a novel structure for the HTS tape.